Apparently Jacob was going to stop, but he said he was led by the Spirit to prophesy. He said that as a result of this stumbling among the Jews, they would not comprehend enough of their own scriptures to recognize the Savior when he came among them. Therefore they would reject him, the very stone which God had prepared for their salvation.
Isaiah had predicted the same thing when he said: "And he [the coming Messiah] shall be for a sanctuary; but [as it turned out it was] for a stone of a stumbling and for a rock of offence to both the house of Israel, for a gin and for a snare to the inhabitants of Jerusalem. And many among them shall stumble, and fall, and be broken, and be snared, and be taken."4 Jacob said this stone was the only foundation upon which the Jews (or anyone else, for that matter) could build and have a safe foundation.
To emphasize his point, Jacob declared that the scriptures plainly teach that Jesus Christ -- or the Stone -- is the great and the last and the only sure foundation on which the Jews can build.